Fiber lasers with high pulse energy, good beam quality and excellent optical characteristics have applications in many fields and industries, such as analytical spectroscopy (e.g., fluorescence, absorption), illumination, remote sensing and environmental spectroscopy (e.g., wind speed, biohazards, eco-system mapping, etc.), ranging and targeting (e.g., collision avoidance, military applications, etc.) and scientific instrumentation. Fiber lasers with exceptionally short pulse widths, for example, femtosecond fiber lasers, have special applications in these and other fields.
There has been great progress in developing short pulse fiber lasers. One approach is to use nonlinearity during amplification in the wavebreaking-free regime of normal dispersion amplifiers to generate a chirped pulse. Pulse compression can then be performed in a coupled section of single mode fiber. U.S. Pat. No. 6,990,270 issued on Jan. 24, 2006 to J. Nicholson and assigned to the assignee of this application is exemplary of this type of arrangement. However, one of the difficulties associated with femtosecond pulses in fibers is compressing the high energy pulses. Nonlinearities in the fiber create distortions in the spectrum, causing the pulse to lose energy to undesirable pedestals or, worse, break up into multiple satellite pulses.
Often, “stretched” pulse amplification is implemented, where the ultrashort pulse is first stretched in the time domain by many orders of magnitude, temporally broadening the pulse and decreasing the peak power. The stretched pulse is then amplified, eliminating or reducing the nonlinear interactions present when attempting to amplify femtosecond pulses. However, whether using a similariton-type amplifier, or stretched pulse amplification, the chirped, amplified output pulse must ultimately be re-compressed, where the high pulse energies associated with amplification means that the recompression stage is usually done using bulk optics.
A fiber that is capable of propagating and compressing high energy femtosecond pulses would then be desirable for two reasons. First, if the fiber can be designed with an appropriate dispersion, it could serve as a compression stage for stretched, high energy pulses. Second, if the compression function can be implemented in a fiber, it can also serve as a delivery fiber for ultrashort pulses for a wide variety of applications, as mentioned above.